To display color images, for example in a CRT screen, the inner surface of a CRT includes three arrays of red, green, and blue phosphors, arranged side by side according to various patterns. The three arrays of phosphors are excited by three very close electron beams deviated together, each of which hits the phosphors of a single color. A scanning control circuit enables deviating the electron beams according to a succession of horizontal lines.
FIG. 1 very schematically and partially shows elements of a screen involved in the image display. Essentially, a horizontal scan control circuit 1 is used with a CRT screen 3 for displaying images based on a video signal (not shown). Circuit 1, synchronized in frequency and phase with the video signal, generates a current I for controlling the deviation of an electron beam 5 emitted by a source (S) 7. Current I has a sawtooth variation between a start of line value −i and an end of line value +i. Current I acts upon a deviator 9 formed, for example, by coils, and determines the deviation of electron beam 5, the intensity of the beam varying according to the video signal. A single electron beam has been shown for clarity, as for a black and white screen. When current I linearly increases between values −i and +i, the point of impact of beam 5 on screen 3 continuously varies between a position A−i and a position A+i. A line is thus displayed on screen. Frame scan means, not shown, enable vertically deviating beam 5 along time, to successively scan all the image lines.
A disadvantage of such a screen is that beam 5 is indeed formed of three electron beams to be very accurately and identically deviated. Now, the sources of these electron beams are arranged to be as close as possible to one another but they are never ideally confounded. Small differences can then not be avoided upon deviation of these beams, which differences generally increase with the deviation angle.
FIG. 2 schematically illustrates the variation of the angle of incidence of an electron beam 5 in the scanning of a line of screen 3. Assuming that the line perpendicular to the screen at a point A0 contains the emission point of source 7, the phosphors located at this position A0 (ideally, at the screen center) are lit with a null angle of incidence, while phosphors located at a position A+i, at one end of the line, are lit with a non-zero angle of incidence α. These errors or deviation defects result in that the three components of beam 5 do not hit screen 3 at a same point. This phenomenon is called a convergence defect and results on screen in a spatial dissociation of the three colors making up each of the points located in the areas where the convergence is not ensured. The presence of these areas is particularly apparent in the case of flat CRT screens, in which the angle of incidence of the electron beams can be high.
To display a correct image, the deviation of the components of electron beam 5 must be individually corrected. This selective correction of the beam convergence can be obtained by a correction means 20 that includes, for example, coils distinct from scanning coils 9. The type and arrangement of such coils, called correction coils, especially depend on the arrangement of the electron beam sources.
FIG. 3A shows, in a front view, three R, G, B sources that generate three electron beams intended for respectively lighting the arrays of red, green, blue phosphors of a color screen (not shown). The R, G, B sources are, in this example, arranged according to a horizontal line parallel to the screen.
A correction means 20 includes four coils 21 having two by two the same axis, which are arranged around the R, G, and B sources along two perpendicular axes that cross at the level of the G source in a plane perpendicular to the beams. Coils 21 are interconnected, and their respective axes form angles of 45, 135, 225, and 315° with respect to the line formed by the R, G, and B sources. The structure illustrated as an example includes four coils 21, but other structures including a larger number of coils also exist. The coils are wound around magnetic cores connected to the internal periphery of a circular magnetic circuit 22. The main field lines between coils 21 have been shown by arrows in dotted lines. The correction is performed by the strongest deviation to which the beams emitted by the R and B sources, closer to the coils, are submitted. According to the direction of the current running through coils 21, the beams emitted by the R and B sources come closer or move away from the beam emitted by the G source. Assuming that beam G is centered, coils 21 enable adjusting the horizontal convergence of the beams emitted by the R, G and B sources.
FIG. 3B shows four other coils 23 of correction means 20. Coils 23 also have the same axis two by two, and are arranged around the R, G, and B sources along two perpendicular axis that cross at the level of the G source in a plane perpendicular to the beams but (in this example) different from the plane in which the axes of coils 21 are inscribed (FIG. 3A). Coils 23 are interconnected and they are arranged so that their axes form angles of 45° with respect to the axes of coils 21. Coils 23 are wound around magnetic cores connected to the internal periphery of a circular magnetic circuit 24. The main field lines between coils 23 have been shown by arrows in dotted lines. According to the direction of the current running through coil 23, the beams emitted by the R and B sources move away or come closer to one another in opposite directions perpendicular to the line formed by the R, G and B sources. Assuming that beam G is centered, coils 23 enable adjusting the vertical convergence of the R, G, and B beams. For clarity, the supply means of the magnetic circuits have not been shown in FIGS. 3A and 3B. Such convergence correction systems are perfectly well known.
It should be noted that magnetic circuits 22 and 24 may be confounded. It is possible to use a combination of coils 21 and 23 to correct convergence problems due to the angle of incidence of the electron beams. Since the angle of incidence varies at each point of the screen, the control signals of coils 21 and 23 must be different for each point of the screen. Further, it is known that, to provide a satisfactory result, the control signals of the correction coils must have as few change of incline points as possible. Indeed, a change of incline of the control signal may in some cases be visible on screen, which is not desirable. Moreover, the control signals provided to the correction coils at a same point of two screens of same type are different, since each screen has specific convergence problems, for example due to the positioning of the sources upon manufacturing of the screen. These convergence problems that vary from one screen to another and above all from one area of the screen to another are called “dynamic” defects, as opposed to “static” convergence defects, which are uniform on a given screen. Such static defects can be corrected by means of coils 21 and 23, for example by application of a D.C. voltage. It should be noted that coils 21 and 23 also enable correcting other dynamic convergence problems, for example problems due to a localized magnetization of the frame. A conventional process consists of generating the control signals of the screen correction coils based on a predetermined number of numerical values measured and stored in the factory for each screen.
A first solution consists of empirically determining the value of the control signal to be provided to the correction coils at a predetermined number of points of several standard screen lines. The values stored for each standard line are provided to an analog filter, which generates a control signal used for the standard line and the neighboring lines. The manual determination of the stored values provided to the analog filter is a lengthy and expensive process, and the number of these values is desired to be as limited as possible. The control signal generated by the analog filter varies between two successive stored values at the rate of the filter loading or unloading. The number of stored values being limited, the time constant of the filter is as high as possible to limit changes of incline of the control signal. In spite of this, the obtained control signal still has changes of incline at the level of its highest points and the number of stored values remains high. Further, many modem display devices are led to pass from one display format to another, which especially implies that the scanning length or duration of a line may vary. The high time constant of the analog filter may be too high if the line scanning duration decreases. Further, the control signals generated for two consecutive standard lines may exhibit strong discontinuities, which is not desirable.
A second solution consists of performing, for example by means of a calculator, a numerical interpolation between the above-mentioned stored values. Such a solution enables generating a control signal curve with no break point. However, this solution has a complex implementation and still requires storage of a large number of numerical values for each line. Further, it is known by those skilled in the art that such an interpolation calculation is delicate to adapt to a change in the line length. For vertical variations, the control signals that are intended for the lines included between two standard lines can be generated by numerical interpolation between the control signals calculated for the two standard lines. Such a method however requires significant calculation resources and storage of a large number of values. Thus, the calculations required by this method cannot, in the state of the art, be performed by a sufficiently powerful calculator due to the scanning speeds involved.